Heart failure has become a problem of epidemic proportions in the United States. For example, it is estimated that more than 10 million people in the United States have some form of coronary heart disease. These patients are at risk of myocardial ischemia, i.e. lack of blood supply to different regions of the heart muscle, and/or infarction, i.e. the death of the heart muscle because of ischemic causes. Myocardial ischemia usually manifests itself with angina (i.e. discomfort in the chest) and this, together with evidence coming from laboratory and other investigational methods, leads to the detection of the ischemia.
However, in a large percentage of these patients, episodes of ischemia are asymptomatic (Silent Myocardial Ischemia) and close to one third of infarctions are silent as well. In addition, silent ischemia might progress to myocardial infarction and myocardial infarction is the most common cause of heart failure and/or cardiac death among heart disease patients.
Current techniques for the detection of ischemia are only moderately sensitive and even less specific, especially in certain subgroups of patients. For example, exercise treadmill electro-cardiogram (ECG) testing is one screening technique, but it is only moderately sensitive and has an unacceptably high false positive rate, particularly in the young and in women. In addition, continuous ambulatory ECG monitoring requires a visit to the physician's clinic or hospitalization and can be difficult to interpret because of the large number of artifacts.
Various implantable devices, i.e. cardiac pacemakers or cardiovertors, have been developed to analyze intracardiac electrograms to diagnose the presence and the evolution of an ischemic state in real time, so as to be able to adapt consequently the operation of the device. An intracardiac electrogram signal collected (i.e. sensed or detected) by electrodes coupled to one or more leads implanted in a patient's heart can be used to monitor a series of wave complexes known as the “PQRST” complexes corresponding to the succession of the cardiac beats of the patient. The QRS complex in a cardiac cycle represents the depolarization of the ventricles and is followed by a T wave which represents the repolarization of the ventricle.
The T wave (repolarization wave) amplitude and shape are quite variable, and are sensitive to conduction disturbances in the myocardium and are therefore, often used to detect and monitor the progression of ischemia. For example, elevation of the amplitude of the T-wave (the ST segment) is a significant indicator of cardiac electric instability of the patient. The level of amplitude elevation of the ST segment can therefore be used to detect and monitor the progression of ischemia. However, the ST segment can be affected by conditions other than ischemia, such as food intake, exercise, diabetes and the like reducing the efficacy of ST segment analysis.